Adamantine – Lustre found in transparent minerals which produces a brilliant bright shine. Example Brazilianite.
Adularescence – Blue/white ghostly reflective appearance, due to structural abnormalities or a build-up of water, term derived from the feldspar variety Adularia. Examples Moonstone and Opal.
Aggregates – The grouping together of crystals. Clusters are examples of aggregates.
Amorphous – No definite crystalline structure or shape. Examples Obsidians and Opals.
Anhydrous – No water molecules are present in the chemical structure. Example Angel Wing Anhydrite (Anhydrite is the mineral Calcium Sulphate) also see Hydrous.
Asterism – A star-like formation of concentrated light, which are inclusions of tiny, slender parallel fibers of minerals that reflect light. Examples are found in Ruby and Diopside.
Aventurescence – The “glistening” effect of reflective minerals such as Hematite, Pyrite, and Goethite. Derived from the word Aventurine. Examples Aventurines and Greenlandite.
Botryoidal – Globular aggregates with a smooth rounded surface that form and appear to be strung together, and which can resemble a cluster of grapes. Examples are Smithsonite, Hemimorphite, and Malachite.
Brecciated – Meaning broken and derived from the rock formation known as Breccia, a sedimentary rock formed through weathering of layers of minerals and organic substances which result in rock fragments being cemented together in a fine-grained matrix. Example Brecciated Jasper such as Youngite.
Cat’s Eye – Dense inclusions of tiny slender parallel fibers which exhibit chatoyancy. Examples are Cat’s Eye Chrysoberyl and Cat’s Eye Tourmaline.
Chatoyancy – Fibres of minerals that exhibit concentrated narrow bands of reflected light across the center of a mineral or crystals and are thus said to be chatoyant. Examples are Selenite, Ruby, and Sapphire.
Concretion – An aggregate composed of a mass of small crystals that have become cemented together resulting in a rounded, ball-like appearance. Example Azurite.
Cryptocrystalline – Also described as microcrystalline, are composed of tiny microscopic crystalline structures that cannot be seen with the naked eye are thus appear to be massive or amorphous. Example Chalcedony.
Crystalline – Crystal structure is formed by the regular molecular arrangement of atoms which are composed of visible crystals that have an atomic structure. Examples of all types of Quartz.
Dendritic – Crystal groups or aggregates of minerals that form either skeletal or branching tree-like formations which are known as dendrites. Can form either on their own or within other crystal formations. Examples are the metals Copper which is dendritic in its formation, Silver which is skeletal in its formation and Dendritic Opal which are dendrites of Manganese within an Opal matrix.
Dichroic – The optical effect known as dichroism (meaning two) whereby one color can be viewed from one angle and another color is viewed in the same mineral from a different angle. Examples are Iolite (Cordierite), Ruby, and Sapphire.
Druzy – An aggregate composed of pointed crystal protrusions from a cavity or base. Also called drusy or druze (druse). Examples are Clear Quartz and Amethyst.
Fibrous Describes an aggregate of a mineral that is constructed of fine, usually parallel threads or fibers. Examples are some types of Tremolite and Malachite.
Fissure – An internal crack or cleavage within a mineral or crystalline matrix. Examples of Fissures within Quartz often produce beautiful “rainbows” as seen in Fire and Ice Quartz.
Fluorescence – A “glowing” effect when a mineral or crystal is illuminated with ultraviolet light. Example Tugtupite.
Fossilized – Organic life from a previous age, the remains of plants and animals that have been embedded or preserved in rock. Examples Fossilized Wood and Turritella Agate.
Gemmy – A mineral that is very transparent and thus considered to be of a high grade. Examples are some forms of Aquamarine and Emerald.
Hydrous – Crystals have water molecules or their elements in their molecular structure. For example, a Gypsum is a hydrated form of Calcium Sulphate. Also, see Anhydrous.
Impurities – Foreign materials often elemental in nature that are not part of a mineral’s integral structure and which can result in color changes within a mineral. Examples include the addition of Manganese to silicon dioxide (Quartz) which results in the purple type of Quartz known as Amethyst.
Iridescence – The light effect which causes a mineral to display a “play of colors” on an apparent mono-colored surface. Examples are the pearly luster seen in many Micas and the metallic luster or tarnishing effect seen in minerals Bornite and Peacock Ore (acid-treated Chalcopyrite) resulting from the oxidation of the metals.
Labradorescence – A mineral displaying dark metallic-like colors that “shimmer” in tones of blue, green, and gold. The term is derived from the gemstone Labradorite which characterizes this color display. Examples Labradorite and Spectrolite.
Lustre – Reflective properties of the surface of crystals and minerals. Adamantine is a brilliant “diamond-like” luster such Brazilianite. Metallic is an opaque reflective luster such as Hematite. Pearly luster as found in Micas. Silky luster is often found in minerals containing fibers such as Tigers Eye. Vitreous is a glass-like luster commonly found in over 70% of minerals such as Quartz (including carbonates, halides, hydroxides, phosphates silicates, and sulfates). Waxy luster whereby a crystal appears to be coated in wax such as Variscite.
Matrix – The name given to a rock or mineral that crystals are found growing on or in.
Opalescence – The color effect was seen in precious Opals, from which the term is derived, which exhibit a glimmer of different colors when rotated or viewed from different angles. Example precious Opals.
Opaque – An optical property of a crystal whereby light is unable to be transmitted through it and thus an object cannot be viewed through it. Examples of various metallic stones such as Hematite.
Petrified – Organic material that has undergone the process of petrification and has been replaced by silica. Example Petrified Wood and Fossil Wood.
Phantom Growth – A phenomenon exhibited when a transparent crystal grows over an existing crystal layer and leaves an inscription of the previous growth within the crystalline matrix. Examples are Phantom Quartz and Angel Phantom or Amphibole Quartz.
Piezoelectric – Substance that generates an electrical charge when under stress. Example Tourmaline.
Pleochroism – An effect present within a mineral exhibiting two or more separate colors when viewed at different angles. Examples are Cordierite (Iolite) which displays blues, purple/blue, yellow, grey, and sometimes red hues, and Alexandrite.
Platy – Crystals and minerals that are small, flat, and flaky in appearance. Examples are Barite and Stilbite which form platy aggregates.
Radiating – An aggregate composed of tiny slender crystals that radiate out from a central point. Examples Scolecite and Stibnite
Reflective – The effect that occurs when light hits a smooth or polished surface and bounces off it. Examples are seen in polished Black Obsidian and Hematite to produce highly reflective surfaces.
Reticulated – An aggregate of long crystals that crisscross each other and form a net or cross-like appearance. Examples are Rutile and Staurolite.
Rosette – Mineral with thin platy aggregates in rounded shapes that resemble the petal formation of rose flowers. Examples Siderite and some Micas.
Rutilated – Inclusions of Rutile within a crystalline matrix. Example Rutilated Quartz.
Schiller – An effect caused by the color reflections or “flashes” present within a mineral. Schiller is German for “twinkle”. Examples are precious Opals and Feldspars.
Spherulitic – An aggregate that has a three-dimensional, circular, ball-like structure that radiates out from a central point. Example Wavellite.
Stalactitic – An aggregate composed of long icicle-like formations such as stalactites. Example some types of Calcite.
Tenebrescence – An unique optical property of certain minerals whereby they change color upon exposure to sunlight and UV light. Example Hackmanite.
Tourmalinated – Inclusions of Tourmaline within a crystalline matrix. Example Tourmalinated Quartz.
Translucent – An optical property of a crystal whereby light is transmitted through it but not fully so that if an object is placed behind the crystals that object will not be clearly seen. Example Chalcedony.
Transparent – An optical property of a crystal whereby light is fully transmitted throughout it, so that if an object is placed behind the crystal that object can be seen clearly through it. Examples are Clear Quartz, Amethyst, and Natural Citrine.
